The present invention relates to communication networks, such as hybrid fiber coaxial (HFC) cable networks, wireless communication networks, satellite networks, etc., in which multiple subscriber stations transmit messages on one or more unidirectional multiple access communication channels. In particular, the present invention relates to enabling each subscriber station to reserve a portion of one or more multiple access unidirectional communication channels for contention free access. This enables the cable network to be used for point-to-point and multicast communication in addition to conventional broadcast TV.
It is desirable to provide ubiquitous, integrated high speed and high capacity digital communication services (such as video, data and voice) to homes, schools, governments, and businesses. One such network, the telephone network, could be upgraded to provide such services. However, the century-old copper telephone network, primarily designed for telephony, has a usable bandwidth of only about 1 MHZ. Therefore, it is quite difficult and expensive to provide multi-channel digital video, along with data and voice on the telephone network. On the other hand, the coaxial drop line of a cable network to each home has a high usable bandwidth of about 1 GHz, providing ample speed and capacity to the integrated broadband services listed above, in addition to delivering traditional broadcast analog video programs. These traditional coaxial cable networks can be readily upgraded to bidirectional hybrid fiber-coaxial cable networks (HFC networks) to enable bidirectional high speed and high capacity communications. The HFC network is inherently a shared medium technology. Nevertheless, providing efficient, high speed, high capacity shared access to the upstream transmission has been a challenge to the communication industries.
FIG. 1 shows a conventional bidirectional hybrid fiber coaxial (HFC) cable network 10 having a head end 12. The head end 12 has a head end controller 28 that can communicate with one or more other networks 30, such as the Internet and local area networks. Downstream directed signals are transmitted from, and upstream directed signals are received at, the head end controller 28 via a coaxial link 34 connected to a diplexer 32. The diplexer 32 splits the downstream directed signals from the other signal carried on the link 34 and outputs them to a laser transmitter 36. The laser transmitter 36 modulates the downstream directed signals onto an optical signal that is transmitted via a downstream optical fiber trunk 14. Likewise, upstream directed signals modulated on a signal carried via an upstream optical fiber trunk 14xe2x80x2 may be demodulated at an optical receiver 38. The diplexer 32 combines such upstream directed signals with the other signals carried on the link 34 for receipt at the head end controller 28.
The upstream and downstream optical trunks 14, 14xe2x80x2 connect the head end 12 to an optical node 16. The head end 12 and optical node 16 may be separated by up to about 80 kilometers. Like the head end 12, the optical node 16 has a laser transmitter 40, an optical receiver 42 and a diplexer 44. The laser transmitter 40 is for modulating upstream directed signals received via the diplexer 44 onto an optical signal for transmission on the upstream directed optical trunk 14xe2x80x2. The optical receiver 42 is for demodulating downstream directed signals from the optical signal carried on downstream optical trunk 14 and transferring the demodulated downstream directed signal to the diplexer 44.
The diplexer 44 outputs onto coaxial trunk 18 the downstream directed signals that are demodulated by the optical receiver. Likewise, the diplexer 44 receives from the coaxial trunk 18 upstream directed signals for modulation by the laser transmitter 40. The individual links of the coaxial trunk 18 are interconnected by bidirectional amplifiers 20 and taps 22. Taps 22 are also provided for connecting coaxial drop lines 22 to the coaxial trunk 18. The coaxial drop lines 22 connect the subscriber locations 26 to the coaxial trunk 18 for upstream and downstream directed communication.
The optical trunks 14, 14xe2x80x2, coaxial trunks 18, taps 20 and coaxial drop lines 22 define a shared communications medium over which communicated signals are transmitted or received by all connected network devices, such as subscriber stations at the subscriber locations 26 and the head end 12. The cable network 10 is specifically designed to deliver information in the downstream direction from the head end 12 to the subscriber locations 26. For downstream directed communication, frequency division multiplexed communication channels are defined which have mutually unique carrier frequencies and non-overlapping bands (6 MHZ bands in North America and other NTSC cable TV systems, 8 MHZ bands in Europe and other PAL and SECAM cable TV systems) in the band from 54 MHZ up to the upper cut-off frequency of the coaxial trunks 18 and drop lines 22 (typically, 500-750 MHZ). This is also known as sub-split cable network. Each 6 MHZ downstream channel can carry either traditional analog NTSC composite video signals or digitally encoded data appropriately modulated by a RF carrier. Each traditional broadcast video programs are each transmitted in a separate communication channel by modulating an NTSC signal onto a predetermined carrier signal having an assigned carrier frequency and transmitting the signal from the head end controller 28.
Although the cable network 10 has a large amount of bandwidth, the cable network 10 presents certain challenges for providing high speed and high capacity upstream transmission from a large number (typically a few hundred) of subscriber locations 26. Most notably, the subscriber locations 26 may be distributed over a large geographic area. The signal path (i.e., sum of the lengths of the coaxial drop lines 22, coaxial trunk links 18 and optical trunk links 14) between individual subscriber locations 26 or subscriber locations 26 and the cable head end 12 can be on the order of tens of kilometers. Such long signal paths introduce noticeable delays in the transmission of signals which tend to be about 5 xcexcs/kilometer.
Recognizing such challenges, several standard bodies and industry consortiums, such as IEEE 802.14, SCTE, MCNS and DAVIC have proposed similar communication schemes as follows. Two channels are defined for communication, namely, an upstream directed channel (UC) and a downstream directed channel (DC). Subscriber stations (SSs) 50 (FIG. 2), such as cable modems, set top boxes or data terminals, at subscriber locations 26 can transmit on the upstream directed channel UC but can only receive on the downstream directed channel DC. The head end 12 can only receive on the upstream directed channel UC and only transmit on the downstream directed channel DC. In other words, the upstream channel UC is a multi-point to point channel whereas the downstream channel DC is a point to multi-point channel. These channels UC and DC are said to be multiple access channels, meaning that multiple network devices (SSs 50, head end 12, etc.) are permitted to access each channel UC or DC. As such, although the physical topology of the cable network 10 is a tree and branch configuration, the communication channels UC and DC may be illustrated as a logical bus network as shown in FIG. 2.
Each channel UC and DC is assigned a different frequency band and center frequency, such as is shown in FIG. 3. As shown, the upstream channel UC may be assigned a band in the 5-42 MHZ band not already used for control message communication. The downstream channel DC may be assigned one of the unused 6 MHZ bands, i.e., not currently used for communicating traditional broadcast video programming. The DC channel is divided into time slots and the UC channel is divided time slots (xe2x80x9cslotsxe2x80x9d) and mini-time slots (xe2x80x9cmini-slotsxe2x80x9d). Point-to-point or multicast communication is achieved by reading packets from, or writing packets into, the slots and mini-slots in a time division multiplexing or time division multiple access fashion. (Herein, a xe2x80x9cpacketxe2x80x9d is an organization of a bitstream into discrete units. A packet may include control or overhead information, typically located in a header section of the packet, and user message or user data information in a xe2x80x9cpayloadxe2x80x9d section of the packet. The term xe2x80x9cpayloadxe2x80x9d is used herein more generally to refer to a channel for carrying communicated data or messages.) In order to read a packet from a channel, the particular channel is tuned (the frequency band of that channel is filtered out of the signals carried on the shared medium), and a packet is demodulated from a respective slot or mini-slot time period of the carrier signal. Likewise, in writing a packet to a channel, a packet is modulated onto a carrier signal of that channel and the modulated carrier signal is transmitted at the appropriate slot or mini-slot time period of the carrier signal and combined with the other signals carried on the shared medium.
It should be noted that the cabling distance (i.e., signal path) between any two SSs 50 and the head end 12 or the mutual cabling distance between any two SSs 50 can widely vary in the cable network 10. As such, a wide disparity of propagation delays may be incurred by each signal transmitted to or from an SS 50 depending on its relative distance to the head end 12. Assuming that the SSs 50 are synchronized to a system clock at the head end 12 using a time-stamping technique (to be detailed later), a packet transmitted at xe2x80x9cthe same timexe2x80x9d from different SSs 50 will arrive at the head end 12 at different times. The difference can be on the order to tens of xcexcsec. If not properly compensated, a large guard time must be inserted between each packet transmission, resulting in a very inefficient time division multiplexing (TDM) transmission in the upstream channels. To overcome this problem, the following procedure, which is generally known, will be described. Each SS 50 is polled and transmits a signal to the head end 12. The head end 12 records the propagation delay of each SS 50. The head end 12 then informs each SS 50 of how long a propagation delay is incurred by signals transmitted from that specific SS 50 to the head end 12. Each SS 50 is also informed of the maximum propagation delay of all SSs 50 in the cable network 10. Whenever a SS 50 decides to transmit a signal, the SS 50 determines the slot or mini-slot boundary at which it desires to write its packet. The SS 50 then delays its transmission from the slot or mini-slot boundary for a certain time period equal to the difference between the propagation delay of the transmitting SS 50 and the maximum propagation delay in the cable network 10. The net effect is that all signals received at the head end 12 xe2x80x9cappearxe2x80x9d to incur the same propagation delay as the SS 50 that incurs the maximum propagation delay.
Each SS 50 is assigned a unique identifier or address. Each packet written into each slot contains at least the address of the destination, i.e., the SS 50, which is the ultimate intended recipient of the packet. A SS 50 transmits information to another SS 50 or to the head end 12 by dividing the information into packets and writing the packets into allocated slots of the upstream channel UC. Such packets are broadcasted by the upstream channel UC to the head end 12 which reads each packet from each time slot. The head end 12 examines the destination address in the header of the packet. The head end 12 writes the packet into an available slot of the downstream channel DC. The packets are broadcasted in the downstream channel DC and are read from the slots by each SS 50. Each SS 50 compares the destination address of the received packets to its assigned address or to the group (multicast) addresses assigned to the multicast groups to which the SS 50 has subscribed. If the addresses match, the packet is accepted. Otherwise, the packet is discarded.
As will be described in greater detail below, two types of packets are transmitted in the channels UC and DC, namely, xe2x80x9cpayloadxe2x80x9d packets and xe2x80x9ccontrolxe2x80x9d packets. Payload packets carry user messages or user data to be communicated to a destination. Control packets carry control messages for allocating portions of the communication channels or other overhead control information. For reasons described below, SSs 50 write control packets into mini-slots of the upstream channel UC and write payload packets into slots of the upstream channel UC. The head end 12 writes payload and control packets into slots of the downstream channel DC. For example, each slot of the downstream channel DC accepts a frame which includes one payload packet and one control packet. This is possible because only the head end 12 writes control and payload packets into slots of the downstream channel DC.
Some manner must be provided to prevent each SS 50 from attempting to write packets into the same time slot of the upstream channel UC. To that end, a slot assignment-reservation protocol is implemented according to which each SS 50 may only write packets into slots that have been assigned to that SS 50. Each SS 50 can attempt to reserve slots (i.e., request an assignment of one or more slots) by writing a reservation request control packet into a minislot of the upstream channel UC allocated for receiving new reservation request packets. The reservation request control packet may indicate the address or identifier of the SS, the number or size of slots needed for the to-be-communicated payload packets, (conventionally, the slot length may be an integral number of mini-slot lengths and thus the number of slots needed may be expressed as the number of xe2x80x9cmini-slotxe2x80x9d lengths needed), the type of the communication for which slots are requested and an error check sequence (e.g., a cyclical redundancy check or CRC). The head end 12 receives the reservation request control packets from the mini-slots and responds by assigning one or more slots to each requesting SS 50. The head end 12 then writes control packets into slots of the downstream channel DC indicating which slots were assigned to each SS 50. Each SS 50 receives control packets that respond to its respective reservation request and then transmits its payload packets only in its assigned slots. Because SS""s 50 only transmit payload packets in their assigned slots, no other SS 50 contends to simultaneously access the same slot. Contention is therefore localized to relatively small size reservation mini slots, and not the relatively lengthy payload packets. Consider that each slot or mini-slot accessed by more than one SS 50 simultaneously (thereby resulting in a collision) is wasted. As such, the use of mini-slots enables the SSs 50 to obtain access to the larger slots in a fashion that conserves the bandwidth.
The payload packets are received at the head end 12. The head end 12 identifies each received payload packet destined to a SS 50 in the cable network 10, and writes each of the identified packets into an available slot of the downstream channel DC. Each SS 50 receives from the downstream channel the payload packets destined thereto.
Nevertheless, contention exists in accessing the mini-slots. Such contention is resolved using a feedback mechanism and a collision resolution algorithm (CRA). The head end 12 monitors each mini-slot and determines if a collision has occurred. If the head end 12 detects a collision, the head end 12 transmits a message via the downstream channel DC indicating in which slots a collision was detected. Each SS 50 that has attempted to transmit a reservation request packet monitors the messages transmitted in the downstream channel DC. If an SS 50 receives a message from the head end 12 indicating that a collision has occurred in the same mini-slot in which the SS 50 had previously attempted to write its reservation request packet, the SS 50 determines that its reservation request packet had collided with another transmission by another device and therefore was not received by the head end 12. In such a case, the SS 50 executes a CRA to determine whether and when to attempt to retransmit its reservation request packet. Several CRA""s are known such as xe2x80x9cternary tree,xe2x80x9d and xe2x80x9cP-persistent and DQRAP.xe2x80x9d See P. Jacquet, P. Muhlethaler and P. Robert, Asymptotic Average Access Delay Analysis: Adaptive P-Persistence Versus Tree Algorithm, IEEE P802.14, Doc. no. IEEE 802.14-96/248 (1996), and U.S. Pat. No. 5,390,181.
It is desirable to reduce contention to increase the utilization of the bandwidth in the upstream and downstream channels UC and DC and, at the same time, accommodate as large a number of SSs 50 as possible. Generally, this is achieved by increasing the ratio of mini-slots to payload slots in the upstream channel UC and decreasing the size of the mini-slots in the upstream channel UC. U.S. Pat. Nos. 5,012, 469 and 5,390,181 describe different variations in the ratio and arrangement of mini-slots to payload slots in the upstream channel UC. The upstream spectrum 5-42 MHZ of a sub-split HFC cable network is susceptible to noises and interference that can limit the amount of spectrum available for reliable transmissions. The noises are, most notably, xe2x80x9cingress noisexe2x80x9d and xe2x80x9cimpulse noise.xe2x80x9d Ingress noise occurs because the coaxial cabling of the trunks 18 and drop lines 22, with imperfect shielding due to corroded connectors, cracked sheath, etc., function as antennas. Different radio transmissions are picked up by the shared medium, such as citizen band (CB) radio broadcasts at around 24 MHZ, short wave radio transmissions at various points in the 5-42 MHZ band, etc., and contribute to ingress noise. Impulse noise, on the other hand, results from noise spikes that occur from other phenomenon such as lightning strikes of the coaxial cabling. The coaxial cabling of the trunks 18 may also carry an electrical power signal for supplying power to the various devices (e.g., amplifiers 20) of the cable network. Power line arching through weak points of the cables and connectors also contribute to the impulse noise.
In order to reliably transmit control packets, such as reservation request packets, in mini-slots, a binary phase shift keying (BPSK) modulation technique or quaternary phase shift keying (QPSK) modulation technique is often used. On the other hand, to maximize the amount of data transmitted in payload packets, a high order quadrature amplitude modulation (QAM) technique such as 16-QAM, 64-QAM or even 256 QAM, with powerful forward error correction (FEC) is often used. However, spectral efficient modulation schemes, such as 16-QAM, 64-QAM and 256-QAM, require longer preambles for carrier recovery and burst synchronization and incur a much higher per burst overhead for mini-slots. That is, each SS 50 actually writes a frame into each mini-slot time period, including an inter-burst guard time period and a preamble, that precede the actual mini-slot control packet, such as is shown in FIG. 4. (FIG. 4 also shows the mini-slot packet structure as including an address or identifier, payload packet or communication type indicator, number of requested mini-slots field and CRC field.) The devices of the cable network 10 may use raised cosine filters. Such filters introduce a ringing into the channel. In addition, transmitters and receivers of the SSs 50 and head end 12 need a finite amount of time to turn on and off in order to read and write packets into specified slots. The purpose of the guard time period is to provide sufficient time for the ringing to dampen and to enable the transmitter or receiver circuitry of the SSs 50 and head end 12 to turn on or off. Following the guard time period is a xe2x80x9cburstxe2x80x9d or combination of a preamble and modulated data. The purpose of the preamble is to enable a receiver to fine tune to the carrier frequency of the carrier signal on which the data is modulated and to align in phase to the carrier signal, prior to sampling the carrier signal and demodulating data from the carrier signal. This synchronization and alignment operation is referred to as xe2x80x9cburst sync.xe2x80x9d Longer preambles are required when spectral efficient, higher order QAM schemes are used to ensure very fine tuning thereby ensuring highly accurate sampling and demodulation. The impact of such effects on mini-slot efficiency are more pronounced as the order of the QAM increases, as depicted in FIG. 5. That is, a larger percentage of the time of the upstream channel UC is allocated to mini-slots as the order of the QAM increases.
To increase the utilization of the upstream channel UC, a technique of varying the time division pattern of the upstream channel UC into mini-slots and slots has also been proposed. This is illustrated in FIG. 6. At the top of FIG. 6, a fixed time division pattern of the upstream channel UC into slots and mini-slots is shown. The disadvantage of this technique is that much of the upstream channel UC capacity must be allocated to mini-slots to account for a typical worst case, or heavy load (numerous attempts to access mini-slots), scenario. In the alternative conventional technique, the ratio of mini-slots to slots can be dynamically varied by rearranging the pattern according to which the upstream channel is time divided into slots and mini-slots. This is depicted at the bottom of FIG. 6. For example, when the load is anticipated to be light (few attempts to access mini-slots), the ratio of mini-slots to slots is reduced. When the load is anticipated to be heavy, the ratio of mini-slots to slots is increased. However, this technique has the following disadvantages:
(1) It is complex to implement.
(2) It is difficult and imprecise to predict the load based on past history, thereby risking a potential stability problem.
(3) It imposes additional constraints on the mini-slot, such as requiring that slots lengths be equal to an integral multiple of mini-slot lengths, further reducing the utilization of the upstream channel UC for payload data.
Although prior art (including those proposed in the emerging standards, such as IEEE 802.14, SCTE, MCNS and DAVIC may include multiple upstream channel support, each upstream channel is statically assigned to the station and each channel is still required to support both the control and payload bitstreams. Such a network still exhibits the inefficiency, high network latency and large delay of a single upstream channel.
U.S. Pat. No. 5,278,833 describes a wireless network including a base station and xe2x80x9ccommunication units,xe2x80x9d such as cellular or cordless phones. This patent describes the circuitry and communication formats in detail. Therefore, only certain details of this wireless communication system are repeated herein. A frequency division multiplexing technique is used to form two channels, namely, an upstream channel having a first band, and a downstream channel, having a second, non-overlapping band. As above, the upstream channel is used for transmitting information from the communication units to the base station and the downstream channel is for communicating information from the base station to the communication units.
Like the cable network 10, a time division multiplexing technique is used to divide each of the upstream and downstream channels into time slots. Each of the time slots may be assigned by the base station for communication between a selected communication unit and the base station. Unlike the cable network 10, the upstream channel is divided only into uniform sized time slots. However, whenever a time slot of the upstream channel is not used for ordinary payload communication, it can be divided into two or more equally sized sub-slots for transmitting control information. A communication unit can communicate by transmitting a request packet in one of the sub-slots of a time slot not previously assigned for payload communication. The base station receives such request packets, determines how many time slots are necessary for the communication unit to communicate, and transmits a control packet in a time slot of the downstream channel indicating which slots are assigned to the communication unit. The communication unit then transmits its packets in its assigned time slot. No contention resolution protocol is specified for transmitting reservation requests. Nor does this patent explain how a communication unit determines that a time slot of the upstream channel is not assigned for payload communication. Finally, note that the upstream channel cannot carry both reservation request packets and payload packets simultaneously. The upstream channel capacity is therefore allocated to each of these kinds of packets thereby reducing the utilization of the upstream channel for carrying payload information.
U.S. Pat. No. 5,012,469 discloses a satellite communications network. The satellite communications network includes plural earth stations that communicate with a satellite station. The communication is bidirectional using a single contentious channel. The channel is time division multiplexed according to one of a number of different formats depending on the traffic load. According to one format, under certain circumstances, the channel is divided into xe2x80x9clarge slotsxe2x80x9d which include one payload time slot and a fixed number of mini-slots. Each mini-slot is uniquely assigned to the earth stations for writing reservation request packets (requesting reservation of payload time slots) for transmission to the satellite station. Under other circumstances, the channel is divided into payload time slots only, and the payload time slots are uniquely assigned to each earth station. As circumstances, such as the traffic load, change, the channel is formatted according to the appropriate one of the two formats. According to a second format, the channel is formatted in one of three different ways, including the two formats mentioned above and a third format in which the channel is divided into time slots which are accessed by the earth stations in a contentious fashion. Again, the channel is formatted according to one of the three different formats depending on the circumstances. In addition to the disadvantages mentioned above for the wireless and cable networks, the architecture suggested in this patent is highly complex.
It is an object of the present invention to overcome the disadvantages of the prior art.
This and other objects are achieved by the present invention. Illustrative environments of use of the present invention are a wireless network, a satellite network, a cable network, etc. In a cable network, a head end is provided as a central controller, a shared medium is provided and multiple stations, namely, subscriber stations, are connected to the head end via the shared medium. Illustratively, the head end transmits one or more traditional broadcast video programs by modulating them onto one or more carrier signals and transmitting the carrier signals on the shared medium. Such traditional broadcast programs may thus be contemporaneously received at each subscriber station. Frequency bands not used for traditional broadcast video programming are assigned for providing point-to-point or multicast communication.
To provide such point-to-point or multicast communication, according to the invention, three types of communication channels, namely, one or more upstream payload channels, one or more upstream control channels and one or more downstream channels, are allocated. Illustratively, the carrier signal of the channels furthermore have mutually non-overlapping bands. Each bitstream is furthermore illustratively organized into packets.
The multiple access network can be formed by assigning the channels with channel identifiers and complete descriptions of the channel profiles, such as carrier frequencies, symbol rates, burst parameters, etc., so that the stations and central controller can communicate. Together with a network ID, a network configuration control message is transmitted by the central controller to all the stations attached to the medium. A minimum of three channels are needed to define the network (DCPC, UCC and UPC). Additional channels can be added to the network by the central controller. Any change in configuration is communicated to the stations using network configuration messages. Further, all control messages to the stations are uniquely identified by the network ID, channel ID, station ID and mini-slot ID, allowing flexibility to either increase or decrease the network capacity and performance according to the needs of the network service providers.
Each channel illustratively is divided into slots or mini-slots. Each upstream payload channel is assigned for carrying upstream directed payload bitstreams from the stations to the central controller. Each upstream control channel is assigned for carrying upstream directed control bitstreams, such as reservation request bitstreams requesting reservation of time slots of the upstream payload channel, from the stations to the central controller. Each downstream channel is assigned for carrying at least downstream directed control bitstreams, such as bitstreams containing acknowledgments and also containing indications of assigned slots in the upstream payload channel, from the central controller to the stations. Each downstream channel illustratively also carries the collision status of collided reservation request mini-slots. The downstream channel may also illustratively carry payload bitstreams.
Illustratively, stations may write reservation request bitstreams into mini-slots of the upstream control channel. Such reservation request bitstreams are received by the central controller, which responds by assigning specific slots to each station. The central controller writes control bitstreams in the downstream channel indicating the slot assignment which are received by the respective stations that issued the reservation request bitstreams. Each station then writes its payload bitstreams only in assigned slots of the upstream payload channel. Illustratively, the payload bitstreams are received by the central controller. If the received payload bitstreams are destined to a station in the network, the central controller writes such payload bitstreams into slots of the downstream channel. Each station receives the payload bitstreams transmitted in the downstream channel, accepts the bitstreams destined thereto, and discards each other payload bitstream.
According to one embodiment, a station communicates on a shared medium of a network as follows. The station transmits a bitstream containing a request to reserve one or more slots of an upstream payload channel on an upstream control channel. The station then receives multiple bitstreams from a downstream channel, including at least one bitstream containing an indication of one or more slots of the upstream payload channel assigned to the station for transmitting packets. The station then transmits payload bitstreams on the upstream payload channel, but only at the assigned slots of the upstream payload channel. In this embodiment, the bitstreams are carried simultaneously on the upstream control channel and the upstream payload channel of the shared medium during overlapping time periods.
According to another embodiment, a central controller of a network enables communication of bitstreams from a station via a shared medium of the network as follows. The central network controller receives from an upstream control channel, a reservation request bitstream, requesting reservation of slots for a particular station. The central network controller transmits on a downstream channel, a bitstream including an indication of one or more slots assigned to the particular station. The central network controller receives a bitstream from one of the assigned slots of an upstream payload channel. Again, in this embodiment, the bitstreams are carried simultaneously on the upstream control channel and the upstream payload channel of the shared medium during overlapping time periods.
By transmitting reservation request packets and upstream directed payload packets on separate xe2x80x9csimultaneousxe2x80x9d channels, each channel can be utilized to its fullest potential. For example, different modulation techniques can be used on each channel, such as BPSK, QPSK, n-QAM, orthogonal frequency division multiplexing (OFDM), discrete multi-tone modulation (DMT), discrete wavelet multi-tone modulation (DWMT), code division multiple access (CDMA), synchronous code division multiple access (SCDMA), etc. This maximizes efficiency of the upstream payload channel yet ensures high reliability and short mini-slot size on the upstream control channel. By reducing the mini-slot size, the likelihood of collision on the upstream control channel decreases, and retransmission delays in the event of collisions can be overall reduced (depending on the collision resolution technique utilized). Likewise, by removing mini-slots from the upstream payload channel, the channel utilization for payload packets is maximized, even while using higher spectral efficiency modulation techniques. Thus, the competing demands of reservation request packets and payload packets can be satisfied without detriment to each other.
In a further embodiment, reservation request and payload packets are not transmitted simultaneously but sequentially, since they are transmitted by a single upstream programmable RF transmitter. In this embodiment, a first switch switches between forwarding modulated UPC and UCC signals to a single frequency agile tuner, while a second switch switches between forwarding an indication of the selected carrier signal f2 and f3, respectively.
The multiple access method can simplify multiple channel support for expanded bandwidth demand and can maximize the number of supported subscriber stations. To increase the capacity, each upstream payload channel or each downstream payload channel can be added to the network. To minimize contention and lower the access delay of the multiple access network, the stream of mini-slots in the upstream control channel can be enhanced by allocating a wider bandwidth, or assigning some subscriber stations to different upstream control channels.